There are known conventional multi step index optical fibers, as disclosed in Japanese Unexamined Patent Publication No. 10 (1998) -111414. Multi step index optical fibers comprise at least two concentrically arranged core portions, each having a constant refractive index. The refractive indices of the core portions decrease from a central core portion to core portions at the outer periphery of the optical fiber.
The refractive indices of the plurality of core portions of these multi step index optical fibers are generally set such that they change along a so called G power distribution. In the G power distribution, the refractive index N at a position at radius R from the core center can be approximated by the following formulaN=Nc·{1−2·δN·(R/Rcore)G}1/2  (1)
wherein:
Nc is the refractive index of the core center,
δN is the difference between the refractive index of the core center and the refractive index of the outermost peripheral portion of the core, and
Rcore is the radius of the core.
Note that commonly, the value of G is set at approximately 2, in order to realize high bandwidth. In addition, the core portions in conventional multi step index optical fibers are formed such that the differences in radii among adjacent cores are uniform (a so called uniform core radius pitch configuration).
It is thought that high bandwidths approximating those of graded index optical fibers may be realized in the aforementioned conventional multi step index optical fibers, by increasing the number of steps of refractive indices, thereby smoothing the refractive index gradient. However, the maximum bandwidth obtainable with conventional multi step index optical fibers is about 1 GHz/100 m. It is considered to be difficult to obtain bandwidths on the order of several GHz/100 m with conventional multi step index optical fibers.
The present invention has been developed in view of the foregoing circumstances. It is an object of the present invention to provide a multi step index optical fiber which is capable of obtaining a bandwidth exceeding 1 GHz/100 m and on the order of several GHz/100 m.